1. Field of the Invention
This invention relates to improved processes for anaerobic digestion of waste materials, and particularly waste materials including significant concentrations of solid material. The invention combines anaerobic treatment techniques with novel anoxic gas flotation processes for separating solids from liquids in the digested waste. The operating efficiency of the process is substantially improved as a result of recirculation of solids containing living anaerobic microorganisms or "biomass" to the anaerobic digester to maintain high concentrations of anaerobic microorganisms therein thus speeding the breakdown of organic compounds to water, methane gas, and other products of anaerobic digestion.
2. Description of Related Art
The anaerobic digestion process has been utilized to treat and remove organic compounds from waste products such as sewage, sewage sludge, chemical wastes, food processing wastes, agricultural residues, animal wastes, including manure and other organic waste and material. As is well known, organic waste materials are fed into an anaerobic digestion reactor or tank which is sealed to prevent entrance of oxygen and in these airfree or "anoxic" conditions, anaerobic bacteria digests the waste. Anaerobic digestion may be carried out in a single reactor or in multiple reactors of the two-stage or two-phase configuration. See, S. Stronach, T. Rudd & J. Lester, Anaerobic Digestion Processes in Industrial Wastewater Treatment, 1986, Springer, Verlag, pp. 93-120 for single reactors and pp. 139-147 for multi-stage operations. Heat is normally added to the reactor or reactors to maintain adequate temperatures for thermophilic or mesophilic bacteria which accomplish the breakdown of the organic material. Mixing of the wastes by either mechanical or gas recirculation is normally provided to accelerate digestion.
The products or effluent from anaerobic digestion consist of: (1) a gas phase containing carbon dioxide, methane, ammonia, and trace amounts of other gases, such as hydrogen sulfide, which in total comprise what is commonly called biogas; (2) a liquid phase containing water, dissolved ammonia nitrogen, nutrients, organic and inorganic chemicals; and (3) a colloidal or suspended solids phase containing undigested organic and inorganic compounds, and synthesized biomass or bacterial cells within the effluent liquid. The liquid phase (2) and the solid phase (3) comprise the effluent mixed liquor from the digestion reactor.
The importance of maintaining high concentrations of anaerobic microorganisms or biomass within the anaerobic digestion reactor has long been recognized. The anaerobic bacteria are responsible for the breakdown or degradation of organic compounds to water, methane gas, and other products of anaerobic digestion. The greater the concentration of these bacteria the more rapid and efficient the process of digestion. This has lead to the use of anaerobic filters which maintain an active biomass attached to inert particles within the anaerobic reactor. See, Midwest Plan Service, Livestock Waste Management With Pollution Control, 1975. Immobilized biomass within ceramic materials has also been used to attempt to accomplish this goal. See, S. Stronach, T. Rudd & J. Lester, Anaerobic Digestion Processes in Industrial Waste Water Treatment, 1986, Pub. Springer, Verlag, p. 145.
Maintaining a high biomass or bacterial population within the anaerobic reactor results in significantly reduced reactor detention time and thus smaller reactor size. Reduction of reactor size results in a reduction in capital costs, as well as energy requirements for heating and mixing of the reactor. Maintaining a high biomass also increases the treatment efficiency since the larger bacterial population reduces or breaks down more of the organic material being processed. Higher treatment efficiencies cause the generation of a greater amount of methane gas. Higher treatment efficiencies also result in a cleaner liquid effluent which can be disposed of more economically and a reduced volume of solids within the effluent which reduces the quantity of material to be transported to an ultimate disposal site. In addition, higher treatment efficiencies or greater destruction of organic compounds, render the solid and liquid effluent more amiable to separation of valuable resources within the effluents such as heavy metals and nutrients such as ammonia nitrogen.
Maintaining a high biomass concentration has also been recognized as providing a more stable anaerobic digestion environment since the high concentration of bacteria are less likely to be influenced to a great extent by minor changes in temperature, pH, and other environmental reactor conditions. See, S. Stronach, T. Rudd & J. Lester, Anaerobic Digestion Processes in Industrial Wastewater Treatment, 1986, Springer, Verlag, pp. 136-138.
As stated previously, anaerobic filters, sludge blanket anaerobic reactors, or encapsulated bacteria in ceramic material of various types maintain high biomass concentrations and thus are able to achieve the benefits listed above when treating certain types of waste products. See, Midwest Plan Service, Livestock Waste Management With Pollution Control, 1975. However, these processes cannot be utilized in the processing of waste containing significant concentrations of suspended solids or colloidal material since such solids will plug the filters and/or porous medium.
In order to overcome the inadequacies of anaerobic filters and encapsulated bacterial retention systems in treating organic materials with suspended solids content, the prior art has identified the use of the so-called anaerobic activated sludge process, which has also been called the anaerobic contact process. See, S. Stronach, T. Rudd & J. Lester, Anaerobic Digestion Processes in Industrial Wastewater Treatment, 1986, Springer, Verlag, pp. 93-120, 136-147. The anaerobic activated sludge process is similar to its aerobic counterpart, the aerobic activated sludge process, which is commonly used to treat industrial and municipal wastes aerobically, i.e., using air with oxygen. Fair, Geyer & Okun, Water Purification and Wastewater Treatment and Disposal, Vol. 2, 1968; U.S. Environmental Protection Agency, Process Design Manual for Suspended Solids Removal, 1975, pp. 7-23 through 7-29.
The aerobic activated sludge process consists of an aerobic reactor wherein a mixed liquor of influent waste and recirculated sludge or microorganisms are maintained under aeration followed by a liquid/solids separator which separates the suspended solids and microorganisms for recycle to the aerobic reactor. The effluent from the liquid/solids separator is substantially free of suspended solids and the vast majority of the influent organic wastes.
The aerobic activated sludge process is commonly utilized as a waste treatment process because it can achieve high degrees of organic waste removal in small reactors containing high concentrations of microorganisms or biomass. High biomass concentrations are maintained by recirculating the solids from a liquid/solids separator. Although many liquid/solids separating processes are available, gravity separation or gravity clarification are primarily utilized. Fair, Geyer & Okun, Water Purification and Wastewater Treatment and Disposal, Vol. 2, 1968, pp. 35-17 through 35-22. Vacuum flotation and dissolved air flotation has also been utilized. U.S. Environmental Protection Agency Process Design Manual for Suspended Solids Removal, 1975, pp. 7-23 through 7-27.
The anaerobic activated sludge, or anaerobic contact process, has not been effectively utilized because the bacteria in anaerobic digestion are not easily separated from the mixed liquor effluent. The difficulty has been that actively fermenting organisms do not settle by gravity because of the buoying effects of attached gas bubbles and the fact that the density of the bacteria closely approximate the density of water and do not floc easily. The use of other common liquid/solids separators also have disadvantages. The use of gravity clarification with the addition of high concentrations of flocculating or coagulating chemicals is expensive and harmful to the bacteria. Rapid temperature and pH changes have also been attempted and found to be harmful to the bacteria. Centrifuging has been found to be expensive and detrimental to the bacteria. Conventional dissolved air flotation as well as froth and foam flotation techniques are detrimental to the anaerobic bacteria since even minute amounts of oxygen or air are sufficient to destroy the bacteria. S. Stronach, T. Rudd & J. Lester, Anaerobic Digestion Processes in Industrial Wastewater Treatment, 1986, Springer, Verlag, pp. 35-38.